1. Field of Invention
This invention relates to wrought nickel-base superalloys with improved creep and stress rupture resistance and, in particular, to Ni--Cr--Co alloys solid solution strengthened by Mo and/or W, and precipitation hardened by the intermetallic compound gamma prime (.gamma.') which has a formula of Ni.sub.3 Al,Ti (and sometimes Nb and Ta).
2. Description of the Prior Art
Steady advances over the years in the performance of the gas turbine engine have been paced by improvements in the elevated temperature mechanical property capabilities of nickel-base superalloys. Such alloys are the materials of choice for the largest share of the hottest components of the gas turbine engine. Components such as disks, blades, fasteners, cases, shafts, etc. are all fabricated from nickel-base superalloys and are required to sustain high stresses at very high temperatures for extended periods of time. As engine performance requirements are increased, components are required to endure higher temperatures and/or stresses or longer service lifetimes. In many cases, this is accomplished by redesigning parts to be fabricated from new or different alloys which have higher properties at higher temperatures (e.g., tensile strength, creep rupture life, low cycle fatigue, etc.). However, introduction of a new alloy, particularly into a critical rotating component of a jet engine, is a long and extremely costly process (many years and multiple millions of dollars today). Material property improvements can sometimes be achieved by means other than changing the basic alloy composition, as for example heat treatment, thermomechanical processing, microalloying, etc. These types of changes are considered less risky and can be made for substantially lower cost and much more quickly.
In the area of microalloying, the positive effect of Boron (hereinafter referred to as B) in nickel-base superalloys has been known since the late 1950's, R. F. Decker et al. in Transactions of the AIME, Vol. 218, (1961), page 277 and F. N. Damana et al. in Journal of the Iron & Steel Institute, Vol. 191, (1959), page 266 demonstrated significant improvements in rupture life for nickel-base alloys from small B additions of 0.0015% to 0.0090% by weight. Phosphorus (hereinafter referred to as P), on the other hand, is an almost unavoidable element which is present in many metallic raw materials commonly used in the manufacturing of nickel-base alloys. There is relatively little published information on the effect of P in nickel-base alloys, and what is available is somewhat contradictory. For the most part, P has been considered to be a harmful, or at best, relatively innocuous element and is controlled to relatively low maximum limits (e.g., 0.015% P and B max. in specification AMS 5706H). Recent work, however, has shown that in certain superalloy compositions, P can, in fact, be beneficial to creep and stress rupture properties. See Wei-Di Cao and Richard L. Kennedy, "The Effect of Phosphorous on Mechanical Properties of Alloy 718", Superalloys 718, 625, 706 and Various Derivatives, 1994, edited by E. A. Loria, TMS, pages 463-477. P is extremely difficult to remove in most pyrometallurgical practices and, in fact, is not changed at all in normal, commercial vacuum melting practices used to produce the alloys of this invention. Therefore, the only means of control of P is to limit the amount in the starting raw materials. With the normal variations in raw material lots, this typically leads to analyzed contents in a commercial nickel-base alloy such as described in AMS 5706H (trade name WASPALOY.RTM., registered trademark of Pratt & Whitney Aircraft) of 0.003% to 0.008%, well within specification limits. To achieve ultra-low P contents, as required in this invention, mandates the use of special, high purity raw materials which are available, but at substantially higher costs or perhaps very specialized melting practices.
Prior to this invention, there has been no recognition of the benefits of producing nickel-base superalloys with such ultra-low P contents (&lt;0.0030% P, or more preferably &lt;0.001% P), and since commercial specifications of &lt;0.015% P have been comfortably met with normal commercial raw materials, there has been a disincentive to produce alloys with very low P. However, it has been discovered that ultra-low P contents (&lt;0.003%, or more preferably &lt;0.001%) when employed in conjunction with higher than normal B levels (0.004% to 0.025%, or more preferably 0.008% to 0.016%) result in significantly improved creep and stress rupture life.